Different fonts for Edit and View modes

### ⚠️ This issue respects the following points: ⚠️

- [x] This is a **enhancement/feature request**. Not a [bug or question](https://github.com/gsantner/markor/issues/new/choose).
- [x] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it and checked most recent issues)_.
- [x] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [x] The wanted feature/enhancement is not present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor's settings are not touched. If your desired feature is present, you don't need to open this issue. The change will be part of the next Markor update.)

### Description

It should be possible to set the font for View Mode separately from the font for Edit Mode. The typical case that comes to mind, and the one I am personally dealing with, is wanting to create a note in some markup language, for which a monospace font could be better suited, and then wanting to view it rendered, for which a usual system font would be better suited.

Currently, the option in the Edit Mode settings controls both fonts, and no analogous option can be found in the View Mode settings. Beside the fact that this is not ideal, that an option in the Edit Mode settings should affect the view mode is rather odd and confusing.

The "Inject" option is not sufficient because it is only viable to some users. Not everyone who can write in Markdown necessarily knows CSS.

It would be better to have a dedicated selection menu like in the Edit Mode settings. For those who want them to be the same, there could be a switch to keep them synced.

Something similar should also be done for font size, since having different fonts for the different modes would require some users to use different font sizes due to the different legibility of different fonts.

### Information

App Version: 2.16.1-1606

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell

```

Start by tracing the existing Edit Mode font and font-size settings, then compare them with the View Mode settings and rendering path. The work is done when View Mode has independent font and size selections, with an optional way to keep them synchronized, while existing behavior remains available.

Written by the indexing model from the issue text.